var mergeTwoLists = function (l1, l2) {
    if (l1 === null) {
        return l2;
    }
    if (l2 === null) {
        return l1;
    }
    if (l1.val < l2.val) {
        l1.next = mergeTwoLists(l1.next, l2);
        return l1;
    } else {
        l2.next = mergeTwoLists(l1, l2.next);
        return l2;
    }
};

let l1 = {
    val: 1,
    next: {
        val: 2,
        next: {
            val: 4,
            next: null
        }
    }
}

let l2 = {
    val: 1,
    next: {
        val: 3,
        next: {
            val: 4,
            next: null
        }
    }
}

console.log(mergeTwoLists(l1, l2));